I bought an AEM Dry Flow for my injen at Advance auto parts way back when i bought the intake. It was about $45 as some have said. It's slightly shorter than the injen filter, but the filter diameter and the neck diameter are the same as the injen filter. I had no fitment problems and recently just cleaned the filter with the AEM cleaning solution from summit. It smelled like watered down Orange-Glo.
And just to add to the thread, i havent had a single issue with my Injen, it's never rattled or bumped anything in the engine bay, it sounds great, adds lots of power, and has never caused a CEL.